The present invention relates to plastics optical fibers and to a process for producing the same. Such fibres, which can more particularly be index gradient fibres, are used for optical transmission, particularly in the medical, telecommunications, automobile and decorative fields.
These optical fibres are generally formed by a first material constituting the fibre core and a second material coating the first to form a fibre sheath or cladding. The core material in which the useful light energy is propagated, must have good light transmitting characteristics and has a refractive index higher than that of the or cladding sheath material.
In such optical fibres, the light is propagated by total reflection at the core-sheath interface. With this type of propagation, the light quantities which can be transmitted increase in proportion to the difference between the refractive indices of the core material and the sheath material.
Glass or silica are generally used as the core material, because they have good optical transmission properties. However, these fibres have the disadvantage of being expensive, heavy and not very flexible. Moreover, over the last few years, various attempts have been made to manufacture optical fibres from plastics.
The hitherto known plastics fibres are formed from transparent materials of the amorphous type, such as e.g. polymethyl methacrylate or PMMA, which is generally used as the core material.
Thus, PMMA has a high transparency and good flexibility, but has a relatively low refractive index (1.48-1.50), which makes it necessary to use a sheath material with a low refractive index. However, polymers with such a low index are extremely rare. One of the polymers which can be used is 2,2,2-trifluoroethyl polymethacrylate, which has a refractive index of approximately 1.41.
Optical fibres formed with this material and the sheath material and PMMA as the core material as well as their production process have been described in Japanese Pat. No. 56 8321 (1981).
Unfortunately, these optical fibres have a low transmissible light aperture angle. In addition, the quality of the core-sheath interface, where the total reflection takes place, is not very good, as a result of the production process used (coextrusion of double die).